Flow characteristics of a control valve As we all know, control valve is automatically in direct contact with the actuator fluid. For the thermal object, the flow and pressure of the control fluid (often water) are related to the realization of the technical goals of automation such as production process and air conditioning. Correct choice of the structure of the control valve, flow characteristics and product specifications for the control system stability, economic rationality has a very important role. Common control valve seat and butterfly valve two categories. With the development of production technology, the structure of regulating valve is more and more. The choice of regulating valve structure is mainly based on process parameters (temperature, pressure, flow), medium properties (viscosity, corrosiveness, toxicity, impurity status) Regulation system requirements (adjustable ratio, noise, leakage) to determine the comprehensive consideration. Under normal circumstances, should be preferred ordinary single, double seat valve and sleeve valve. Because of the simple structure of such control valve, spool shape easy to process, more economical; or according to the specific requirements of the appropriate selection of the structure of the control valve. After the determination of the structure type, the specific specifications of the regulating valve relate to whether the flow characteristics of the valve match the characteristics of the system, and whether the stability of the system is high or not, and the economy is good. Regulating valve flow characteristics, refers to the relative flow of fluid flowing through the control valve and the relative opening of the control valve. It can be easily deduced that the relative flow is positively related to the relative opening, that is, the smaller the valve passage, the smaller the relative opening, the smaller the relative flow rate. The larger the valve passage is, the larger the relative opening is and the larger the relative flow rate is. Valve channel is zero, then the flow is zero, the valve is closed. From the fluid mechanics we can see that the flow through the valve and the pressure before and after the valve into a positive correlation, that is: Where: Q refers to the flow through the valve; ΔP is the differential pressure formed before and after the valve; K is the coefficient. The pressure difference is often determined by the fluid passage formed by the valve opening (spool displacement L). The smaller the opening degree is, the smaller the relative opening degree is and the greater the pressure difference between the front and the back of the valve. The larger the opening degree is, the larger the relative opening degree is , The smaller the pressure difference before and after the valve. It can be said that the size of the flow through the regulating valve is not only related to the opening of the valve but also to the pressure difference before and after the valve. Regulating valve in the work, when the valve opening changes, not only the flow has changed, before and after the valve pressure has changed. In order to facilitate the discussion, assume that the pressure difference before and after the valve a certain, that is, to discuss the ideal flow characteristics, and then consider the actual situation in regulating the valve in the pipeline, that is, discuss the characteristics of the work flow. 2 ideal flow characteristics Ideal flow characteristics of the valve before and after a fixed pressure drop under the flow characteristics obtained, it depends on the shape of the spool, also known as the structural characteristics. In the ideal case, the flow rate changes with the valve opening changes, from the control point of view, the control valve to observe the indicators, the study of flow characteristics, is a commonly used method. Among the commonly used control valves, there are four typical ideal flow characteristics, as shown in Figure 1 [1]. 2.1 Linear Characteristics The relative flow of the control valve is in a linear relationship with the relative opening, as shown in (1) in Figure 1. Curve slope unchanged, that is, its amplification factor unchanged. Take the relative travel as 10%, 50% and 80% as an example. When the travel is changed by 10%, the relative change is 10%, and its relative change (ie sensitivity) is 100% and 20% respectively. 12.5%. It can be inferred that in the case of the same change of travel, when the relative opening of the valve is small, the relative flow change value is large and the sensitivity is high; when the relative opening degree is relatively large, the relative flow change value is small and the sensitivity is low. This tends to degrade linear control valve performance: at small opening, the amplification factor is relatively large and the adjustment process tends to oscillate; at large opening, the amplification factor is relatively small and the sensitivity is low, making it easy to Valve slow, adjust the time extension. 2.2 The logarithmic characteristics of its unit relative stroke changes caused by changes in relative flow at this point is proportional to the relative flow rate, as shown in Figure 1 (2) curve. The same trip L is equal to 10%, 50%, 80% of the three as an example, when the trip changes 10%, the flow changes were 1.9%, 7.4%, 20.5%, we can say that the amplification factor with the valve open Increase. Therefore, this valve in a small opening, the amplification factor is small, work to ease and smooth; large opening, the amplification factor, work sensitive and effective. Similarly, the sensitivity of each point is 40% everywhere (also known as equal percentage characteristics), easy to control. 2.3 Fast-opening characteristics and parabolic characteristics Fast-opening characteristics As shown in curve (3) in Fig. 1, when the valve opening is small, the flow rate changes greatly. As the opening degree increases, the flow rate reaches the maximum value rapidly, the amplification factor is large, high sensitivity. When the valve opening is large, the flow does not change much, the amplification factor is smaller, and the sensitivity is also lower. In the pressure is not too big, less demanding regulatory requirements of the application, open fast, off is slow, not easy to cause large pressure pipe network fluctuations. Parabolic characteristics shown in Figure 1 (4) curve. The relative flow caused by the change in unit relative stroke of this valve is directly proportional to the square root of the relative flow at this point. It lies between curves (1) and (2), and its characteristics are close to logarithmic valve characteristics, but it is less used due to its complex spools. 3 flow characteristics When the control valve in the process piping system, the resistance changes in the pipeline system or the degree of opening of the bypass valve before and after the valve pressure changes, making the same valve opening, no longer like the ideal flow characteristics That traffic will remain the same, the corresponding traffic will change. We call the flow characteristics of the control valve before and after the pressure changes as the operating characteristics. 3.1 The characteristics of the work flow in tandem pipeline In the project, the control valve is installed on the pipeline system with resistance, shown in Figure 2. When the total pressure difference between the two ends of the system is fixed, the pressure drop on the regulating valve will decrease as the flow rate increases [2]. As the valve opens, the differential pressure across the valve decreases. Therefore, with the same valve opening, the flow at this time is smaller than the ideal flow rate. When the valve opening is larger, the pressure difference between the control valve and the front and rear decreases, and the flow rate is larger. In Figure 2, ΔP is the total pressure difference of the piping system, ΔP1 is the pressure difference of the regulating valve, and ΔP2 is the pressure difference across the pipeline and equipment. Let S = (ΔP1m / (ΔP), where S is the weight coefficient of the valve, and ΔP1m is the pressure drop across the control valve when the valve is fully open.When the valve is not changed and different pipeline resistance is changed, its S value Is different.As the pipe resistance increases, the value of S decreases.Under different S values, for the ideal characteristics of the linear and equal percentage flow characteristics of the valve, the operating characteristics shown in Figure 3 [3]. 3 shows that when S = 1, that is, the total system pressure acts on the control valve, and remains constant, then the ideal characteristics.With the S value decreases, the flow control valve full-opening decreases, but in a relative opening (Q = Q / Q100, Q100 is the flow when the valve is fully open when there is resistance in the pipe.) Thus, operating characteristics occur relative to ideal flow characteristics Distortion, as a group of upward arch of the curve.Thus, in a small opening, the amplification factor is larger, higher sensitivity; large opening, the amplification factor is smaller, the sensitivity is lower.At the same time, if we compare the relative The flow at zero opening is called the minimum flow, and the minimum flow and the maximum flow Q100 ratio of the reciprocal called adjustable ratio, then with the S value decreases, due to the impact of the series pipeline resistance, valve adjustable ratio becomes smaller.It can be inferred that the adjustable ratio R and the valve weight of a large relationship : Where R is the adjustable ratio of the ideal flow characteristics, called the ideal adjustable ratio; Rs adjustable ratio of the work flow characteristics, called the actual adjustable ratio. The smaller the adjustable ratio, the regulatory capacity of the regulating valve Low; Adjustable ratio is greater, then the regulation of the control valve the stronger, but the actual adjustable ratio relative to the ideal adjustable ratio, can not be too large, because to consider the system's energy consumption, under normal circumstances, S 0.3 ~ 0.5 [4], the actual adjustable ratio control in the ideal adjustable ratio of 0.55 ~ 0.70 3.2 When the parallel pipeline flow characteristics of work Figure 4 is the case of parallel control valve. Although both ends of the control valve pressure Constant, its parallel bypass valve opening degree will also affect the flow characteristics of the control valve.If Q100 that the control valve is fully open when the flow through the control valve, Qmax said the maximum flow of the manifold to x to represent the bypass Degree, then. Under different x values, its Flow characteristics as shown in Figure 5 [5] .Figure shows, when x is equal to 1, the bypass valve is closed, the flow control valve is the ideal flow characteristics of the flow characteristics of the bypass valve with the gradual opening of the bypass valve The flow rate increases, the value of x decreases continuously, the flow characteristics do not change, but the adjustable ratio greatly decreases.The relationship between the actual adjustable ratio and the bypass degree x is that there is always the influence of the tandem pipeline in practical application, The adjustable flow of the valve becomes very small, and even the regulating valve can hardly play a regulating role. In general, it is desirable that the minimum value of X is not less than 0.8 [6], so that the maximum flow of the regulating valve is 80% of the total flow, The curve is close to the ideal characteristic, and the adjustable ratio R can not reduce too much. For the linear valve, the sensitivity is lowered when the opening degree is small to avoid the oscillation phenomenon. For the logarithmic valve, the amplification factor is smaller when the opening degree is small, and the sensitivity change of the entire stroke tends to be constant, with an approximate equal percentage characteristic, and the quality of the adjustment can still be maintained. For quick-opening valves and parabolic valves, the operating characteristic curves have the same tendency to change and should be used when using. It should also be noted that the flow with (1-x) Qmax can not be regulated when working in parallel as this part of the flow out through the bypass valve. From the control point of view, when the relative opening of the control valve is relatively small, the relative flow rate is small, relative to the ideal characteristics, the control valve is retarded, the adjustment time is prolonged, and the adjustment ability is decreased. 4 control valve stability analysis Valve in the practical application of the role of the system, only discuss the control valve itself or simply discuss the relationship between the valve and the system is not enough, we should conduct a holistic analysis. Generally speaking, the whole system can be divided into two parts: the adjustment system and the object to be transferred. The former includes the measuring sensor, the regulator and the actuator (the actuator includes the adjusting mechanism, the regulating valve and the heater). Taking the temperature as an example, the signal connection among the various components is shown in FIG. 6. In general, the measured signal is tuned → Comparator → Regulator → Regulator → Regulator → Heating (Cooling) → Slave → Called → Repeated cycle Objects in the task of being measured. From the point of view of the transferred object, most of the thermal objects respond to the exponential decay law with the step signal, as shown in Figure 7 [7]. In the process of transition, the object to be tuned relative to its input signal, the amplification factor Kc is not a constant, often from small to large direction changes. From the regulation system, in addition to heaters and control valves, the control of other components can be reduced to a proportional amplification factor of proportionality [8]. For hot water heaters, as the relative flow rate increases, the temperature difference between the heated fluid and the inlet decreases, and the relative temperature rise decreases. Its static characteristics are shown in Fig. 8 [9]. It can be seen that its amplification factor decreases with the increase of relative flow rate, which is not a constant. In this way, with the exception of the control valve, for the entire system, the total system amplification factor is smaller with the load increases, and in a relatively small period of time (transitional process time), the total amplification factor is increasing over time of. This has a great influence on the quality of the system adjustment. If the total amplification factor of the control loop remains constant over the entire control system [10], it is of great benefit to the stability of the system. In the actual production process, due to the non-linear characteristics of the object to be adjusted and the heater, the amplification factor of the control loop should be selected considering this factor. Therefore, the appropriate choice of regulating valve characteristics, to adjust the valve's amplification factor to compensate for the control object amplification factor changes, the system's total amplification factor set to maintain the quality of control in the entire operating range to maintain a certain. If the object to be adjusted and the heater characteristics for the linear nature of the valve can be used linear operating characteristics to ensure that the adjustment system in the operating range of approximately linear characteristics of the total system amplification factor is a constant. For most of the thermal objects and hot water equipment, their amplification factor is smaller as the load increases, we can choose to enlarge the coefficient with the load increases and the larger the logarithmic characteristics of the control valve, two Just exactly compensate each other. This total system amplification factor is also a constant, help to improve the stability of the system. Adjusting the nonlinearity of the dynamic characteristics of the object and the device, it is difficult to ensure the stability of the total amplification factor of the system only by the straight line and equal percentage characteristics of different diameters. For more complex situations, consider the parabolic nature of the valve and other difficult control valve, it is necessary to consider the characteristics of the appropriate regulator to ensure the stability of the total amplification factor. From a control point of view, the stability of the increase, often will cause the rapid decline of the system, the accuracy will decline. We can choose high-performance regulator and regulating valve work together to shorten the transition process time, in order to improve the system's rapidity. At the same time, try to keep the control sensitivity within the operating range as well: not too big, so that the system will not oscillate violent movements; not too small, the system adjustment time is shortened. If coupled with the accuracy of system design, the system will reach the "sensitive and accurate, stable and rapid" advanced level of control. 5 Conclusion Adjust the valve in a small relative opening work, the higher the sensitivity, easy to make the system moves frequently, affecting the quality of regulation; in the large relative opening work, the sensitivity is low, the amplification factor is small, the system is not easy to stabilize. In the field of thermal engineering to seek: ①try to make the system pressure is constant at both ends, the system tends to ideal job characteristics, easy to control. ② the coefficient of control valve larger, increase the adjustable ratio, to improve the regulatory capacity. ③ the characteristics of the valve and the object to be adjusted and the dynamic characteristics of equipment phase compensation, the system in the operating range of amplification factor stability. In this way, the entire control system can reach a certain level of advanced control.
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